14 September 2015

MIL 188-110A/B/C 150bps, 66.6mS ACF

This is a MIL 188-110A waveform: decoder k500 (and others) recognizes this signal just as a 188-110A and 150bps/S data-rate. The curious is that this signal doesn't exhibit the 200mS ACF value but a strong 66,67mS frame (160 symbols) and an equally strong "superframe" of 200 mS length that meets the expected value (200mS). Me and Angazu tried to clrafify the reasons of this.

ACF of this signal (150 bps data-rate)

Let's have a look at the 188-110 protocol and frame formations:

The reason is that the standard 188-110A/B/C has always a 200 mS ACF, but in case of low data-rate speeds (from 150 up to 1200 bps) four groups of the pairs probe+data count 160 symbols (4 x 40) and they are just in sync with the scrambler length (160 symbols) causing the strong 66.67mS ACF.

66.6mS frame (note the 20+20 groups for low data-rate)

MIL 188-110A/B/C 150 bps

In case of 2400 bps (data and/or voice) we'll have groups of 48 symbols (32 symbols for data +16 symbols for probe) and this value is not correlated to the the scrambler length, so the "visible" ACF in this mode (and in 4800 uncoded) is the known 200 mS.In case of the lowest speed (75bps) there are no channel probe so the 66.6mS ACF is just due to the scrambler length (160 symbols).

MIL 188-110A/B/C 600 bps

MIL 188-110A/B/C 2400 bps

"So...we can state that ACF is 200 mS, but scrambler and group formation size generate the 66,6 mS ACF in user speeds of 1200Bd and lower" Angazu says.A signal with these same features (modulation, speed and ACF) is also reported in radioscanner.ru site but named as USAF MIL 188-110A Compatible and nothing more precise.